Commits on Sep 11, 2024

1. Deadlock between Watcher and Debounce threads (fix parcel-bundler#187) …

   There are two threads involved in `Watcher.cc` and `Debounce.cc` each
   calling into respective methods of the other and thus each potentially
   holding onto locks of the other. This can lead to a deadlock in the
   following scenario:
   
   While the `Debounce.cc` thread is processing callbacks in the
   `Debounce::notify()` method, it holds its own lock. The method loops
   over callbacks to process in `Watcher.cc` which itself requires a lock
   in `Watcher.cc`. If an event gets reported while the debouncer is in
   `Watcher.triggerCallbacks()`, a deadlock is present, because:
   - assume the event thread is thread A
   - assume the debouncer thread is thread B
   - A holds its own lock in `Watcher::notify()` and calls into
     `Debounce.trigger()` which requires the debouncer lock
   - B at this time is inside `Debounce::notify()` holding its own lock
     processing callbacks and about to call into `Watcher.triggerCallbacks()`
   - A deadlocks waiting for B to release the debouncer lock
   - B deadlocks waiting for A to release the watcher lock

   

   bpasero committed Sep 11, 2024
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